Polyimide resins are widely used as resin varnish for semiconductor interlevel dielectric or surface protective film due to their excellent properties including heat resistance, flame retardance, mechanical properties and electrical insulation. In general, polyimide resins in varnish form in solvents are applied to semiconductor devices directly or via a dielectric film and cured to form a protective film of polyimide resin, after which the semiconductor devices are further encapsulated with epoxy resin or similar molding materials. The resulting packages have the problem that different coefficients of expansion among the components, semiconductor member, substrate and encapsulant allow thermal stresses to generate during subsequent thermal cycling and solder reflow steps, which can invite chip cracks and thermal deterioration. It is thus desired to reduce the modulus of elasticity of polyimide resin so that the protective film may absorb the stress. One approach is to incorporate siloxane linkages into the polyimide resin skeleton for providing a lower modulus of elasticity. Regrettably, the cured film of the resulting resin has a lower glass transition temperature (Tg), leading to a decline of heat resistance.
Nowadays, packages are being reduced in size and profile, and the surface mount type packaging becomes the mainstream packaging technology. Under the circumstances, many prior art epoxy resin compositions fail to maintain satisfactory reliability. As the solders are also tailored to be lead free, the solder reflow temperature is increased to 260° C., which gives rise to the trouble that packages having absorbed moisture will crack during soldering or even if no cracks generate, moisture resistance will decline. It would be desirable from this standpoint as well to have a protective film with better heat resistance and quality.